Communications over the VHF medium is limited to line of sight restrictions, i.e. the transmitter and the receiver must be within clear line-of-sight to communicate with each other. Any obstruction by building or terrain may attenuate the VHF signal significantly and cause a communications interruption. As VHF is the primary mode of air/ground data communications, the system must be able to overcome these path blockages to provide reliable connectivity between the aircraft and the system of ground stations (GS).
Category-A Aircraft Communications Addressing and Reporting System (ACARS) is a connection-less, broadcast data link. All ground stations within the line-of-sight of an aircraft capture each downlink message and forward it to an intended destination. Although this approach creates multiple copies of the same message on the ground network, it effectively addresses the VHF path blockage problem.
Category-B ACARS is connection-less but point-to-point data link. The aircraft detects a received signal strength indicator (RSSI) of all ground stations within line-of-sight and directs downlinks to a specific GS having the highest signal strength. Only the addressed GS receives and processes the downlink. If the aircraft receives no response to its downlink, it directs the transmission to the next best GS as determined by the RSSI. Therefore, Category-B ACARS eliminates the multiple copies in the ground network at the cost of added delay due to re-transmission of downlinks when no response is received from the selected GS. The aircraft traverses through the coverage region of a GS very rapidly, typically within 10 to 12 minutes. Therefore, the optimum GS as determined by the RSSI is expected to change frequently, which causes the downlinks to be directed to different stations. Both ACARS systems being connectionless, there is no overhead associated with switching the transmission from one GS to another.
The next generation data link, called VHF Digital Link (VDL), is currently being implemented. VDL is a connection-oriented system whereby the aircraft must establish a virtual connection with a specific GS before any user data can be exchanged air-ground. Similar to Category-B ACARS, VDL uses the received signal quality parameter (SQP) to select the optimum GS with which to communicate. The connection establishment process requires an exchange of information between the aircraft and the chosen GS, which adds delay and protocol overhead.
Unfortunately, the REF environment changes rapidly for an aircraft moving at a high speed, which is several hundred miles per hour for commercial aircraft. As a result, the instantaneous nature of the received signal strength makes it a poor metric for selecting the optimum GS with which to communicate. Some GS selection implementations combine distance estimation with RSSI/SQP to improve the GS selection algorithm, but do not compensate for rapidly changing RF environment. This approach has limitations. Aircraft and ground station position information is not always available to compute the distance. Also, this approach may select a less optimum ground station from RF overhead perspective in situations when the aircraft is moving away from the selected ground station. In this case, the aircraft will be within the VHF coverage region for a shorter period of time and connection to a new ground station must be established sooner. Such frequent ground station switching reduces effective RE bandwidth available for transferring user data because connectivity changes require additional messages to be exchanged over the RF.